HUMAN BIOLOGY

366 Chapter 18

how Meiosis produces new Combinations of genes

are rearranged. Remember that germ cells contain sets of

homologous chromosomes, one set that came originally

from a person’s mother and a corresponding set from the

father. These homologues therefore can be called “maternal”

and “paternal” chromosomes and they carry genes for the

same traits. During meiosis I, the two pairs of sister chroma-

tids (one maternal, the other paternal) line up very closely.

This close alignment favors crossing over. In a cross-

over, nonsister chromatids break at the same places along

their length and exchange corresponding segments. The

X-shaped areas shown in Figure 18.12 are crossovers. Each

segment in the exchange includes one or more genes.

The exchange of chromosome

pieces is called genetic recombina-

tion. It leads to variation in the

details of inherited traits because

a gene may have several chemical

forms. For example, as described

in Chapter 19, the gene for earlobe shape has two forms, one

for attached earlobes and the other for detached earlobes.

Often, particular forms of genes on one chromosome differ

from corresponding ones on its homologue partner. When a

crossover occurs between chromosomes in a germ cell, both

then have a slightly different version of their genes than

they had before. If a “Mom” chromosome had the gene for

attached earlobes and the “Dad” had the gene for detached

earlobes, the situation may now be reversed.

gametes also receive a random assortment

of maternal and paternal chromosomes

As you know from looking at Figure 18.11 in Section 18.7,

during metaphase I of meiosis the maternal and paternal

chromosomes get lined up and tethered to the spindle in

preparation for the formation of two new daughter cells. The

chromosomes line up at random, making it highly unlikely

that one daughter cell will receive only maternal chromo-

somes and the other will receive only paternal ones. In fact,

odds are that there will always be a mix of maternal and

paternal chromosomes in each cell that forms during meio-

sis. Figure 18.13A shows the possibilities when there are only

three pairs of homologues. In this case, eight combinations

(2^3 ) of maternal and paternal chromosomes are possible for

the forthcoming gametes.

Of course, a human germ cell has a full 23 pairs of

homologous chromosomes, not just three. So a grand total

© Cengage Learning

n    Events that happen during meiosis explain why no person is

genetically identical to either parent.

pieces of chromosomes may be exchanged

No one ever looks, or has a body that operates, exactly like

his or her parents. Most of the trait variations we take for

granted result from changes to chromosomes that occurred

during meiosis, when germ cells were forming sperm in a

father’s testes or eggs in a mother’s ovaries.

Some genetic variations come about during prophase I

of meiosis. This is a time when parts of the duplicated

chromosomes—the sister chromatids—in gonad germ cells

18.8

crossing over Swapping

of corresponding segments

between homologous chro-

mosomes during meiosis.

Figure 18.12 Animated! During meiosis I, a process called crossing
over may create new combinations of genes on the duplicated
chromosomes in germ cells. For clarity, this diagram of a cell shows
only one pair of homologous chro mosomes and one crossover. Here,
the paternal chromosome is blue and its maternal homologue is pink.

© Cengage Learning

B

A

B

A

b

a

b

a

Bb

AA

B

a

b

a

A Here, we focus

on only two of the

many genes on a

chromosome. In

this example, one

chromosome has

genes A and B; the

other has genes a

and b.

C Crossing

over mixes up

paternal and

maternal forms

of genes on

homologous

chromosomes.

B Close contact

between homologous

chromosomes pro-

motes crossing over

between nonsister

chromatids. Paternal

and maternal chro-

matids exchange cor-

responding pieces.

One duplicated

chromosome

One duplicated

chromosome

B

A

B

A

b

a

b

a

Bb

AA

B

a

b

a

A Here, we focus

on only two of the

many genes on a

chromosome. In

this example, one

chromosome has

genes A and B; the

other has genes a

and b.

C Crossing

over mixes up

paternal and

maternal forms

of genes on

homologous

chromosomes.

B Close contact

between homologous

chromosomes pro-

motes crossing over

between nonsister

chromatids. Paternal

and maternal chro-

matids exchange cor-

responding pieces.

One duplicated

chromosome

One duplicated

chromosome

B

A

B

A

b

a

b

a

Bb

AA

B

a

b

a

A Here, we focus

on only two of the

many genes on a

chromosome. In

this example, one

chromosome has

genes A and B; the

other has genes a

and b.

C Crossing

over mixes up

paternal and

maternal forms

of genes on

homologous

chromosomes.

B Close contact

between homologous

chromosomes pro-

motes crossing over

between nonsister

chromatids. Paternal

and maternal chro-

matids exchange cor-

responding pieces.

One duplicated

chromosome

One duplicated

chromosome

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